The present invention is directed to the field of wireless digital communications. With the recent proliferation of wireless communications, there is an increasing demand for wireless traffic, resulting in increasing channel interference. Consequently, a number of modulation schemes are employed to efficiently utilize the limited frequency spectrum. Such schemes require precise synchronization between the receiver and transmitter in order to decode the signal for extracting the transmitted data.
A well known method of receiver synchronization is to simultaneously transmit a synchronization signal along with the transmitted data. However, such a synchronization signal wastes bandwidth and is susceptible to distortion in the same manner as the transmitted signal. Consequently, it is desirable to recover the synchronization parameters at the receiver directly from the transmitted signal.
Orthogonal frequency division multiplex (OFDM) modulation is a multicarrier modulation method in which several subcarriers are modulated with the desired information. These subcarriers are then simultaneously transmitted by the transmitter. The frequency relationship between the various subcarriers is such that they are orthogonal in the mathematical sense, permitting the receiver to recover the data from each subcarrier. This method allows excellent synchronization, along with excellent bandwidth utilization and performs well in multipath radio frequency (RF) environments.
OFDM modulation suffers from certain drawbacks. Of particular concern is OFDM burst operation at very high data rates. In this case it is very difficult to rapidly synchronize the receiver to the transmitter. Usually, the longer the synchronization sequence the better the quality of synchronization. This however is contrary to the goal of transmitting at high data rates, i.e. sending a large amount of data in a very short time, since the synchronization time is wasted with regard to data transfer. The shorter the synchronization time the better.
Another concern is the latency introduced by synchronization. Latency in high rate OFDM transmission can cause a serious problem when it increases to such an extent that it takes an excessive time to synchronize to and decode the received data. In particular there are specific cases within the IEEE 802.11a standard where very short turn around times are required. (The turn around time is the time difference between a received message and the reply.) There are several processes that introduce significant amounts of latency; data interleaving and convolution/block code decoding. These processes are required and it is difficult to reduce their inherent latencies. Thus it is necessary that the initial synchronization be rapid and robust while incurring no loss in synchronizer performance. This presents a difficult task since better synchronizer performance is achieved with both longer synchronization sequences and latency (i.e. more time to analyze the synchronization sequence), both of which are contrary to the needs in a high data rate OFDM communication system.